Axial piston pumps have been used in various environments and are particularly well suited for use in marine steering assemblies wherein hydraulic lines extend from the steering position on the boat to a rudder or outboard engine for controlling an actuator for pivoting the rudder or outboard engine for controlling the direction of the boat. In a typical situation, a steering wheel, or the like, is connected to a shaft of an axial piston pump whereby, upon rotation of the axial piston pump, hydraulic fluid is directed from the pump to one side or the other of the actuator for steering the boat. Such a pump is shown in the U.S. Pat. No. 4,092,905 to Wood, issued June 6, 1978.
Generally, pump assemblies of the type to which the subject invention relates include a housing having first and second interconnected portions with a cylinder barrel supported in the housing and a plurality of pistons reciprocatively supported in a plurality of piston cavities in the cylinder barrel for reciprocating movement. A cam reacts with the pistons for causing the reciprocating movement as either the cylinder barrel or the cam is rotated. The cam reacts with the pistons to move the pistons between a position where the cavities in which they are disposed are at a minimum pumping volume to an intake position where the cavities are at a maximum intake volume. During the compression stroke of the pistons for creating the pumping action there is a reaction force, because of the pressure of the fluid created by the pistons, which reacts between the pistons and the cylinder barrel forming the piston cavities. These reaction forces are transmitted to the two housing portions which are connected together in a manner to force the housing portions apart or to place the housing portions in tension.
Ball pistons have been used in a variety of situations as parts of fluid pumps. For example, the U.S. Pat. Nos. 735,248 to Hahn, issued Aug. 4, 1903 and 2,095,553 to McGee, issued Oct. 12, 1937, disclose pumps including ball check valves. The U.S. Pat. No. 2,941,475 to Blair, issued June 21, 1960 discloses a hydraulic pump wherein balls engage a cam surface.
Prior art ball piston assemblies included a body having an inlet in the crown of the piston and an outlet at the opposite end. A check valve system is disposed between the inlet and outlet for controlling the fluid flow between the inlet and outlet ports.
In hydraulic pumping systems, particularly hand operated systems used for steering boats, it is important to remove all air from the oil for proper operation. The use of valves, such as the check valves, which direct return oil directly to the tank and inlet check valves in each piston aid in flushing air from the system. The optimum configuration has the inlet check valves in the crown of the piston and outlet at the end of the bore. The piston is always oriented so that the inlet is level with or below the outlet. In this way, air is flushed directly out of the piston bore since there are no traps for the air as in systems with the inlet check valves in the piston bore. Another important aspect of manually operated hydraulic steering systems is the smoothness of operation. Systems containing pistons with hardened or chrome plated balls in the piston crown provide the smoothest operation.
Present designs of pistons containing inlet check valves in the crown have a hole in the center of the crown. This system has several draw backs. The contact point between the piston and the seat or race for the ball is offset from the centerline of the piston which increases side load and reduces efficiency and life of the system. The contact area on the side of the hole in the piston crown does not slide as smoothly on the race as a more conventional round end piston. The hardness and friction characteristics of the material at the point of contact are limited to that of the piston material or expensive plating systems. Since the angle of the race is usually 10.degree. to 15.degree., the inlet area is restricted, thereby increasing the pressure drop and the possibility of cavitation. In order to provide a smooth contact area at the desired angle, the piston end must be machined which adds cost.